I don't use trees now, but I still have some advice.
Buy trees at the time pumu suggested. Get upgrades as early as you can too, scorps without upgrades are useless. Keep a good supply of mana, as I said you want to buy upgrades early, and now scorps cost mana. Don't get too many trees, they cost a ton of pop and you want to keep a lot of room for other spellcasters like infernos or V. About 3 would be optimal. Don't keep your trees holed up in your base, be mobile but cautiously. If you get caught off guard you're toast.
Last thing, this is very important. DON'T tech switch to charrogs. Tech switching is about the worst mistake you can ever make. Maybe buy one charrog to lead the scorps, but do not tech switch to charrogs. I hope this helps, keep practicing!